Semiconductor manufacturing technology has advanced to the point where a single die may contain millions of active devices. As an example, the manufacture of 16 Mb DRAMs is now possible and 64 Mb and 256 Mb prototypes are being developed. A key requirement for achieving such high device packing density is the formation of suitable storage capacitors.
In general, DRAMs are formed with either stacked storage capacitors or trench storage capacitors. Both capacitor structures can be adapted to high density applications. Trench storage capacitors, however, offer some advantages over stacked storage capacitors including a smoother surface topography.
With a trench capacitor, a trench is etched vertically from the wafer surface into a substrate, which is typically formed of silicon. A dielectric material is then formed on the trench sidewalls by deposition (i.e. Si.sub.3 N.sub.4) or oxidation (i.e. SiO.sub.2) and the center of the trench is filled with a conductor (e.g. polysilicon). The final structure is wired from the surface. The silicon and polysilicon serve as the two electrodes of the capacitor with the silicon dioxide dielectric between them.
For high density applications, anisotropic dry etching is a preferred method of forming the trenches. This produces a nearly vertical but slightly tapered sidewall profile.
As the cell size available for capacitors decreases, the surface opening of a trench capacitor also decreases. The surface area available for capacitance thus decreases. To compensate for this loss of surface area, the trench must be etched deeper. A high aspect ratio trench is one that is formed with a depth that is much greater than its width. For high density applications, aspect ratios (depth/width), of at least 5:1 are preferred. Aspect ratios of greater than 40:1, however, have been achieved in laboratory experiments.
Glow discharge dry etching is a preferred method of forming high aspect ratio trenches. With glow discharge dry etching, the wafers are placed in a vacuum chamber and the trenches are etched using a gas mixture containing an etching gas. Typically, the gas mixture is energized to a plasma state using a power source and a radio frequency (RF) field. The gas provides a medium in which a glow discharge can be initiated and maintained. An etch mask formed of a material such as silicon dioxide (SiO.sub.2) is used to pattern the location of the trenches and the size of the trench openings. The trench depth in turn, is a function of the etch rate and the etch time.
A problem with the use of glow discharge dry etching processes to form high aspect ratio trenches, is that the etch rate for cutting the trench is limited. It is theorized by the present inventor that these problems arise because the etching process is influenced by the rate of diffusion of reactant materials into the trench and by the rate of diffusion of byproduct materials out of the trench. These rates of diffusion vary with the depth of the trench.
This situation is illustrated in FIG. 1. In FIG. 1, a trench 10 is being etched into a silicon substrate 12. An etch mask 14 has been deposited over the substrate 12 and includes an opening 15 to define the location of the trench 10. A process gas may be formed as a cracked feed gas that includes reactant materials 22 that react with and remove the unprotected silicon to form the trench 10. In FIG. 1, the reactant materials 22 are illustrated as the triangular shapes. The diffusion of the reactant materials 22 is thermally random but a net diffusion is downward into the trench 10 as indicated by downward arrow 18.
As the trench is formed, a passivated layer 24 of material (i.e. non volatile film) builds up on the sidewalls of the trench 10. Etching ions 16 contained within the process gas aid in the etching of the unprotected silicon and in particular prevent the formation of the passivated layer 24 on the bottom most portion 25 of the trench 10. This allows the reactant materials 22 to continue etching the trench 10 into the substrate 12.
The reaction of the reactant materials 22 with the silicon substrate 12 also produces byproduct molecules 20. The byproduct molecules 20 are indicated by the solid circles in FIG. 1. The diffusion of the byproduct molecules 20 is thermally random but a net diffusion is upward and out of the trench 10 as indicated by upward arrow 19. In general, the reaction mechanics and etch rates for forming the trench 10 are affected by the concentration of the reactant materials 22. In addition, the reaction mechanics for forming the trench 10 are affected by the concentration of the byproduct molecules 20.
To a lesser extent, the reaction mechanics for forming the trench 10 are affected by the thermal diffusion of the etching ions 16 into the trenches 10. In general, however, the concentration of the etching ions 16 is much smaller than that of the reactant materials 22 (i.e. one ion for every 10.sup.3 -10.sup.4 gas molecules). Moreover, because etching ions 16 are accelerated by the voltage between the plasma and substrate 12 into the trench 10, the rate of transport of the etching ions 16 into the trench 10 is not greatly influenced by classical diffusion/concentration mechanics and is not greatly reduced by an increasing aspect ratio.
The concentration of the reactant materials 22 and of the byproducts molecules 20 varies with the depth of the trench 10. In general, the reactant materials 22 are more concentrated at shallow depths close to the surface of the substrate 12, and become less concentrated as the depth of the trench 10 increases. Conversely, the concentration of the byproduct molecules 20 increases with the depth of the trench 10.
This situation is illustrated in the graphs shown in FIGS. 2 and 3. As shown in FIG. 2, the concentration of reactant materials 22 decreases with the depth of the trench. Conversely, and as shown in FIG. 3, the concentration of byproduct molecules 20 increases with the depth of the trench.
This situation limits the etch rate for forming the trench. The etch rate slows as the trench depth increases due to the reduced transport of the reactant materials 22 to the lower portions of the trench as well as the reduced transport of the byproduct molecules away from the lower portions of the trench. Stated differently, the average etch rate decreases with increasing aspect ratio. This phenomena may prevent the trench 10 from reaching the required depth. More importantly, this phenomena may prevent a trench from forming within an acceptable period of time in a production process.
These reaction mechanics cause problems during the large scale manufacture of semiconductor devices. In particular, the decreasing etch rate necessitates longer etch times. The throughput of the etching process step is thus unacceptably low. The present invention is directed to a method of increasing the etch rate of trenches formed in a semiconductor substrate during such a dry etch step.
Accordingly, it is an object of the present invention to provide a method for forming high aspect ratio features in a semiconductor manufacturing process using a glow discharge dry etching process. Yet another object of the present invention is to provide a method for forming high respect ratio features in a substrate in which a rate of formation of the features is improved. It is a further object of the present invention to provide a dry etching process for forming high aspect ratio trenches in a substrate. It is yet another object of the present invention to provide a dry etching process for forming high aspect ratio trenches in a semiconductor structure that is adapted to large scale semiconductor manufacture and in which a throughput of parts during trench formation is improved.